1. Field of the Invention
The present invention relates to a novel discharge lamp in which a high pressure arc tube for high pressure discharge is electrically connected in parallel to a low pressure arc tube for low pressure discharge and both arc tubes are placed in an outer tube. This novel lighting equipment starts the low pressure arc tube and the high pressure arc tube with a high frequency lighting starter.
2. Description of the Prior Arts
In general, the mercury pressure in a high pressure arc tube is about several atm. during the lighting of the discharge lamp in a high pressure mercury vapor discharge lamp, a metal halide vapor discharge lamp or a high pressure sodium vapor lamp which have three to ten times the luminous efficiency of an incandescent lamp. Therefore, when the discharge lamp is once turned off by a temporary power voltage drop or a temporary service interruption, the discharge lamp is not able to start immediately even though the power voltage has returned to the normal level.
It has been difficult to start the lamp until the temperature in the arc tube is lowered to decrease the mercury vapor pressure in the arc tube to the level required for the start. The time required for the restart of the discharge is called the restart time which is about 3 to 5 minutes in the high pressure mercury vapor discharge lamp, 8 to 15 minutes in the metal halide vapor discharge lamp and 1 to 2 minutes in the high pressure sodium vapor discharge lamp which is started with a ballast equipped with a high voltage pulse generator, and 2 to 10 minutes in the high pressure sodium vapor discharge lamp which is started with a ballast having no high voltage pulse generator. Thus, the high pressure discharge lamps which require a long restart period are not satisfactory, in their uses.
When a high pressure discharge lamp having high efficiency is being considered to take the place of the incandescent lamp which is mainly used as a light source but has a quite low efficiency, the disadvantageous long restart period should be considered.
A ballast is required for starting the high pressure discharge lamp, which is large and heavy and accordingly, is assembled separately from the discharge lamp. Therefore, it has been impossible to consider the replacement of the incandescent lamp by a combination of the ballast and the discharge lamp.